1. Field of the Invention
The invention relates in general to an illumination device, and more particularly, to an illumination device used in a display.
2. Description of the Related Art
The conventional projection apparatus has been widely applied in front projection type or large panel rear projection type displays. The required source has to provide enough luminance. Typically, light sources such as halogen lamps, arc lamps, for example, high pressure mercury lamps, metal halogen lamps, xenon lamps are employed. These lamps have the advantage of high luminance, but on the other side, have the disadvantages of high electricity consuming, short lifetime, and high temperature. These light sources are thus selected only while a high luminance is required.
FIG. 1A and FIG. 1B show two types of conventional illumination devices. FIG. 1A is a schematic drawing of a projection apparatus disclosed in U.S. Pat. No. 5,418,583. A first lens array 20 and a second lens array integrator 30 are used to uniformize a light source 10, and to project the light source 10 onto a liquid crystal display (LCD) light valve 40. FIG. 1B is a schematic drawing of a projection apparatus disclosed in U.S. Pat. Nos. 4,656,562 and 5,634,704. A glass rod integrator 25 and a lens 35 are used to uniformize a light source 10, and to project the light source 10 onto a liquid crystal display (LCD) light valve 40. In both of the projection apparatus shown in FIG. 1A and FIG. 1B, a halogen lamp or various type of arc lamp is used as the light source 10. Therefore, the projection apparatus has very high electricity consumption, a short life time, and high temperature and is not suitable for use in a small scale display.
The diagonal length of a desktop display screen is typically in a range of about 20 inch to 30 inch. The required luminance of the light source is not as high as the front projection type or large panel back projection type displays. Moreover, the desktop screen is often in a ON status, so the light source has to keep supplying a light. Therefore, the conventional light source with a high electricity consumption, short lifetime, and a high operation temperature is not suitable to apply in a desktop display screen.
It is an object of the invention to provide an illumination device which employs a low power light emitting device as a light source. The light emitting device has the properties of low electricity consumption, long lifetime, and low operation temperature, that is, low thermal consumption. By superposing several light emitting devices on a light valve, the lifetime and the light intensity of a light source are increased of orders in magnitude. In addition, with the advantage of low operation temperature, the optical lens used in the illumination device can be selected from plastic material, so that the fabrication cost is lowered.
It is another object of the invention to provide an illumination device which includes a uniformizing means and a polarization means to efficiently apply the light supplied by the light source, and to obtain an optimum results of projection.
It is a further object of the invention to provide an image projection apparatus. The illumination device provided above is used as a light source. Therefore, the projection apparatus can obtain a high efficient projection display with a low fabrication cost.
To achieve the above-mentioned objects and advantages, an illumination device and an image projection apparatus using the illumination device are provided. The illumination device comprises at least a light emitting device. A light emitted from the light emitting device is uniformized by a uniformizing means. The light source includes an LED. Being uniformized by the uniformizing means, the light projects on a light valve such as a liquid crystal display to display an image. In the invention, various types of illumination uniformizing means can be employed and are introduced in the section of the detailed description of the preferred embodiments. By the illumination uniformizing means, an incident light can be re-distributed or converted into a way of back light panel to achieve the objective of being uniform.
In addition, most of the light valves can only receive a single type of polarization light. The illumination device thus further comprises a polarization converter to convert a light into a useful polarization type of light. Therefore, the efficiency of the light source is enhanced.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.